This invention relates generally to electrostatography and more particularly to improvements in electrostatographic copying employing liquid development techniques.
Processes for forming latent electrostatic images, existing as electrostatic charge patterns upon a substrate, and for subsequently converting the latent electrostatic image into a visual pattern, are well known. Generally, such electrostatic techniques have been carried out by using toners which are dry powders. However, many techniques have been developed in which the toner particles are incorporated in a liquid carrier, and in electrostatic printing wherein latent electrostatic images are formed on a photoconductive surface of a recording element by uniformly charging the surface thereof, as by a corona discharge device, followed by exposure to light in the desired image pattern, such images may also be developed by liquid developers. In these electrostatographic copying processes a charge pattern is established on an imaging surface and is developed by a liquid development process wherein the liquid developer is presented to the charge pattern by suitable applicator means. In one type of liquid development, the suspended toner particles are electrostatically charged and develop the latent image by migration of the particles to the image surface under influence of the image charge. This is known as electrophoretic development and utilizes the developers having insulating liquids of relatively high volume resistivity. In another type of liquid development, the entire liquid developer is attracted to the imaging surface in image configuration by the electrostatic forces of the charge pattern. Liquid developers for these techniques are well known in the art.
One of the preferred types of electrostatic image development is disclosed by Gundlach in U.S. Pat. No. 3,084,043 and U.S. Pat. No. 3,551,146, where liquid developers having relatively low viscosity, low volatility, contrast in color in the usual case to the surface on which it will remain, and relatively high electrical conductivity (relatively low volume resistivity), are disclosed for converting the electrostatic latent image to a visible image. According to this method, liquid developer from a reservoir is deposited on a gravure roller and fills the depressions in the roller surface. Excess developer is removed from the lands between depressions, and as a receiving surface charged in image configuration passes against the gravure roller, the liquid developer is attracted from the depressions in image configuration by the charge. This method of development is referred to as polar liquid development. This type of liquid development process is also described by Amidon et al in U.S. Pat. No. 3,806,354 where high volume resistivity developers are used. In certain embodiments the development occurs on a interposition surface or web which has been imaged while in intimate contact with the photoconductor surface. Liquid developer in image configuration on the interposition surface is then transferred to another substrate. As used herein, "imaging surface" also encompasses interposition surfaces or webs, and both types of surfaces are deemed to be "surfaces to be cleaned".
In the development of electrostatic latent images where liquid developers are used to develop the image on the reusable photoconductor surface or on the reusable interposition surface, liquid developer remains on the surface after the developer image is transferred to a substrate. In these processes and apparatuses where the surface is reused to develop subsequent images, the residual liquid developer must be removed therefrom to such an extent that the residual developer will not interfere with subsequent imaging causing streaks and other image or non-image patterns and smudges of developer which transfer to subsequent substrates. When the developer remains on the surface to be cleaned in levels above transfer thresholds, then transfer of the developer will occur at the transfer station. In order to accomplish sufficient removal of the residual developer so that the streaking and smudging of subsequent prints is eliminated, several prior art methods have been attempted, but they have generally been unsatisfactory.
Cleaning means which are more specifically intended for use in the liquid development processes are shown in copending U.S. Pat. applications, U.S. Ser. No. 482,716 filed June 24, 1974 now U.S. Pat. No. 3,940,282 issued to S. C. P. Hwa; U.S. Ser. No. 409,994 filed Oct. 26, 1973 now abandoned; and U.S. Ser. No. 482,726 filed June 24, 1974 now U.S. Pat. No. 3,918,809; issued to S. C. P. Hwa.
The cleaning means of the prior art include brushes, wiper blades, scraper blades, rotating absorbent members rotating parallel with the surface being cleaned, and sets of blades working together. Scraper blades, also frequently referred to as "leading edge blades ", have in the past been found to be the most satisfactory cleaning means for use in the cyclic liquid development of charge patterns. This type of member is deemed to be one of the prior art "conventional cleaning means".
Wiper blades are also commonly used to remove residual quantities of liquid developers, but cleaning systems comprised of wiper blades leave streaks of developer on the surface, especially when dust, lint, paper fibers and the like collect between the wiper blade and the surface. Thus, when wiper blades are used, frequent cleaning of the blade itself is required to remove the debris collecting on the blade. Debris, defined herein as lint, dust, paper fibers and other undesirable solid particles which collect in copying machines, interferes with the intimate contact between the blade and the surface. Futhermore, when the wiper blade is used in contact with the surface sufficiently intimate to remove the residual developer, there is either excessive wear of the surface especially if the surface is rigid, or there is frequent damage to flexible surfaces, such as, film-like materials.
Dual systems for the removal of liquid developer from copy sheets have been described in the prior art wherein a doctoring apparatus (wiper blades) is used to remove excess liquid from the developed image surface of the copy sheet prior to squeegeeing of the sheet. However, such a system does not remove layers of toner deposited in image configuration on the copy sheet, and accordingly, it is incapable of removing residual streaks and smudges and other image and non-image patterns from such surfaces as photoconductor and interposition surfaces.
Other dual cleaning systems disclosed in the prior art encompass wiper blade assemblies following sponge-like members, but in such systems the sponge-like members are used to apply cleaning solvents, and the wiper blade removes the solvent or prevents solvent from passing to other parts of the apparatus. In view of the limitations on the use of cleaning solvents in electrostatic printing machines due to environmental concerns and health hazards, such techniques are limited in application. Furthermore, in any dual cleaning system for electrostatic copying systems using liquid developers wherein the second cleaning element is a wiper blade, eventual accumulation of debris on the wiper blade will interfere with the ability of the blade to maintain intimate contact with the surface with the result that streaks and smudges of developer will remain on the surface in spite of the fact that the surface is in contact with the blade. Other systems comprising wet cleaning webs have been proposed but these systems utilize solvents to loosen and remove residual toner. Such systems are undesirable because they require solvent applicator systems and generally employ solvents of a hazardous nature which effect environmental purity. Rotating brush systems have also been suggested but brushes contribute to the debris accumulation.
Imaging member cleaning means also suitable for use in the cyclic development of charge patterns include those shown and described in U.S. Pat. No. 3,522,850; U.S. Pat. No. 3,781,107; U.S. Pat. No. 3,728,016; U.S. Pat. No. 3,859,691; and U.S. Pat. No. 3,664,300. Such cleaning means, however, are most suitable for use in "dry xerography" as taught for example, in U.S. Pat. No. 2,297,691 wherein the charge patterns are developed by a finely divided material referred to in the art as "toner". In U.S. Pat. No. 3,728,016 there is shown an open celled foam wiper which is mounted for wiping engagement with the plate for removal of marking material from an imaging surface, the foam wiper being held in engagement with the imaging surface in the form of a pad. The powdered marking material is removed from the imaging surface by the foam and the foam is thereafter cleaned of toner particles by electrical bias and a serrated portion of the imaging plate. However, this system is deficient for cleaning liquid developer from the imaging surface because of the absence of a primary cleaning member and because it is merely at most a conventional foam wiper pad. Furthermore, because of the belt configuration there would be substantial drag from the working surface contacting the imaging surface. The exposed pad material is also deficient because it tends to absorb liquid developer and upon reaching a certain level of saturation, thus type of member reapplies transferable amounts of absorbed liquid developer to the surface being cleaned.
While ordinarily capable of producing good quality images, conventional liquid developing systems suffer serious deficiencies in certain areas. The above enumerated prior art cleaning devices when used in connection with certain oil based liquid developers typified by those described in the prior art, often fail to prevent the formation of streaks or deposits of liquid developer on the imaging surface. These streaks and/or deposits transfer to the final copy even when multiple blades or conventional foam pads are used. The deposits or streaks of liquid developer are observed to appear on the imaging surface after as few as 400 cycles or less in some systems although other systems may not develop deposits or streaks until after 8,000 cycles. Once formed, the deposits and/or streaks build up sufficiently to cause unacceptable print out of the streak or deposit in the final copy after a comparatively small number of additional cycles.